Checking the Yokomo BMax4-III manual, it shows M2x8mm screws to be used on the bevel gear, not Xray's M2.5. Your next problem is that the Xray case is M2.5 - so you'll need to enlarge the screw holes in the steel bevel gear to accept the M2.5 screw. If you have a steady hand g'head - but a drill press is best.
Next - the outdrives are Xray outdrives right? What you need to ensure is that the outdrives stick out from the diff the same amount/distance as the stock diff. I wonder if Yokomo outdrives would work - all of a sudden this conversion is a lot more complicated. You may have to shim a little on the inside and outside of the diff to get the right clearances - might even have to cut the outdrive. Dremel time. Be careful, that's hardened Hudy steel, no beers until after the work is done! Sparks!
Lastly, the Yokomo o-ring set is BD-500GM. It does not show the actual size of the proper o-ring to be used. The Xray uses a 5x2mm o-ring, 972050, so my guess is you'll need the Yokomo kit above (about $7, includes shims, gear and cross pins, noe of which will probably fit...) or simply find some other o-ring, maybe a Kyosho? Is the Xray o-ring too big or too small?
How some pics of your progress, matching the Yok/Xray vs. a stock Xray diff?
Did the pinion fit onto the center CVD ok?
http://www.petitrc.com/setup/yokomo/...oBMax4III.html